Telephone system



Jane 22, .1943. E, VROOM' 2,322,330

I TELEPHONE SYSTEM ,d NORA/Ev June 22, 1943.

MRT? 4 E. vRooM TELEPHONE SYSTEM Filed SQpt. 28. 1940 5 Sheets-Sheet 5WVE/VTOR E. VROOM A NORA/Ey v Patented `lune 22, 1943 TELEPHONE SYSTEMEdward Vroom, Ossining, N. Y., assignor to Bell Telephone' Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 28, 1940,Serial No. 358,820

' (ci. 17g- 8.5)

v claims.

This invention relates to telephone systems and particularly to systemscomprising message-rate, party line subscribers st ations.

Itis the object of the Vinvention to enable the automatic selectiveoperation of the calling subscribers message register on callsoriginating at party line subscribers stations when the call isanswered' and to enable the reoperation of the message registerperiodically during the time the talking connection is maintained.

According to this invention, means are provided in a telephone systemfor selectively operating either of the message registers of a twopartyline, one of the registers being connected to the line Aconductor towhich the winding of the line cut-off relay is connected, for operationby alternating current of one frequency ,and the other register beingconnected to the same conductor of the line for operation by alternatingcurrent of a different frequency. The one or the other of twoalternating current generators is automatically connected in series witha booster battery to this conductor of the calling line when the call isanswered and periodically thereafter, to operate the callingsubscriberfsregister, the booster battery being effective to hold the line cut-offrelay while the register is being operated.

A feature Yof the invention is a telephone system in which means areprovided for automatically, selectively and periodically operatingeither of `the message registers o f a two-party line, each of theregisters being connected -by a three-element gas-filled tube to theline conductor to which the winding of the line cut-oli relay isconnected. Means are provided for connecting a booster battery potentialto this conductor to hold the line cut-01T Vrelay operated, and forsuperimposing on the boosterbattery potential an alternating potentialof one frequency or an alternating potential of another frequency, thealternating potential of one frequency being effective to causebreakdown of one tube and the alternating potential of the otherfrequency being effective to cause breakdown of the other tube, thebooster battery potential being effective in either case to maintain theenergization `of the energized tube and operatively energize theassociated message register.

According to anotherfeature of the invention means are provided in atelephone system for automatically, selectively and periodicallyopnected. This conductor is also connected to a harmonic generator whichis effective, in response to the application of an alternating potentialof one frequency to the conductor, to generate a harmonic which isapplied through a tuned circuit to break down one of the tubes; and, inresponse to the application of alternating potential of anotherfrequency to the conductor, to generate a harmonic which is appliedthrough another tuned circuit to break down the other tube; .boosterbattery being connected to the conductor in either case to hold the linecut-off relay, to maintain the energization ofthe energized tube, and tooperate the associated message register.

The drawings, which form a part of this specication, show schematicallysystems and circuit arrangements in which the invention and its variousfeatures vare embodied. The invention is, however, not limited in itsapplication to the sys- `tems and circuit arrangements shown, but isgenerallyapplicable to any message rate, party line, communicationsystem.

Referringto the drawings:

Fig. 1 shows an vautomatic telephone system comprising subscribersstations PI and P2 connected by a line Ll to a line circuit LCI and toterminals in the banks of a line-nder switch LF and a Vconnector switchCI, amessage rate trunk circuit MRT! vconnecting the line finder 'LF toa rst selector switch. Sil, another .connector switch C2 connected toterminals jin the bank of selector Sl, and a line La: connecting thestation X to terminals in the bank of the connector C2;

Fig. 2 shows two subscribers stations PI and P2 connected by a line L2to a line circuit LC2 and to terminals in the banks of a line-finderswitch LF and a connector switch C, the line circuit LC2 being anYalternative to the line circuit LCI in Fig. 1;

Fig. 3 .shows two subscribers stations PI and P2 .connected by aline L3to a line circuit LC3 and to terminals in the banks of a line-finderswitch LF and a connector switch C, the line circuit LC3 being anotheralternative to the line circuit LCI in Fig. 1; vand Fig. 4 vshows anautomatic telephone system comprising two subscribers" stations Pl andP2 connected by a line L4 to a line circuit L04 and to terminals in thebanks of a line-lnder switch LF and a connector switch CI, a messagerate trunk circuit MRT2 connecting vthe line finder LF to a rst selectorSl, another connector switch C2 connected to terminals in the bank ofselector S I, anda line L5 connecting the station Y to terminals in thebank of connector C2.

The subscribers stations PI and P2 in each of the figures of thedrawings, the station X in Fig. 1 and the station Y in Fig. 4 are eachof the common battery type which includes a dial or other impulse senderfor controlling the operation of selector and connector switches throughwhich connections with called subscribers stations are established. Eachof the stations PI differs from the station P2 `connected to the sameline in that there is no connection of ground potential to theconductors of the associated line upon removal of the receiver atstation Pl; Whereas removal of the receiver at each of the stations P2connects both conductors of the associatedline to ground through theringer'ZD, as shown in Fig. 1. This difference enables identification ofthe calling station on each call initiated over each of the party linesLI, L2, L3 and L4, as hereinafter explained.

The line-iinder switches LF, selector switches SI, and connectorswitches CI and C2 are of the well-known two-motion step-by-step type.Reference may be had to Automatic Telephony, 2nd edition, by Smith andCampbell, pages 53 to 65. for a description of the structure of suchswitches and their operation when used as selectors and connectors.Reference may be had to the patent to R. L. Stokely No. 1,799,654 ofApril 7, 1931, for a description of the operation of a two-motionstep-by-step line finder.

The line circuit LCI comprises a line relay 30, a line cut-off relay 35,two message registers MRI and MR2, one for each of stations PI and P2,and two three-element gas-filled tubes TI and T2, each of which, whenenergized, operatively connects the associated one of the registers tothe conductor 33. Operation of the line relay 30, upon initiation of acall at either one of stations PI and P2, is effective as described inthe aforementioned patent to R. L. Stokely to start an idle line nder,the cut-off relay being operated as soon as the calling line isconnected by a line finder to an associated message rate trunk and firstselector. Relay 35 locks in usual manner to holding ground potentialconnected, through the test brush of the line finder, to'conductor 34.Relay 35 also connects conductor 34 to the anode 4elements of tubes TIand T2` and'through a harmonic generator comprising condenser 36,inductance 31 and inductive winding 38, to ground. 'Ihe harmonicgenerator is of the type shown in Fig. 1 of Patent No. 2,117,752 grantedto L. R. Wrathall May 17, 1938, and is adaptedto generate odd harmonicsof each of two frequencies, for instance, 60 cycles and 120 cycles, uponapplication of alternating potentials of these frequencies to conductor34, as hereinafter explained. Condenser I4 and the primary winding oftransformer I3 constitute a circuit tuned to an odd harmonic of one ofthe two frequencies for applying a breakdown potential across thecontrol elements of tube TI in response to the application of analternating potential of the one frequency to conductor 33, ashereinafter described. Condenser 24 and the primary winding oftransformer 23 constitute a circuit tuned to an odd harmonic of theother of the two frequencies for applying a breakdown potential acrossthe control elements of tube T2 in response to the application of analternating potential of the other of the two frequencies to conductor33, as hereinafter described. While any odd harmonic might be used forbreakin`g down the tubes TI and T2, a higher harmonic, for instance, thethirteenth, permits the use of smaller and more economical elements inthe tuned circuits through which the breakdown potentials are applied tothe tubes. The breakdown of tube TI operatively connects register lVLRIto conductor 33 and the breakdown of tube T2 operatively connectsregister MR2 to conductor 33.

The line circuit LC2 is similar to the linecircuit LCI except that thetuned circuits for generating harmonics of the two frequencies areconnected in series instead of in parallel with each other.

The line circuit LC3 is similar to the line circuit LCI except that theharmonic generator is omitted, the condenser I4 and transformer I3 beingeffective to apply a breakdown potential of a first frequency, forinstance 750 cycles, to tube TI, and the condenser 24 and transformer 23being effective to apply a breakdown potential of a second frequency,for instance 1500 cycles, to tube T2.

The line circuit LC4 is similar to line circuit LCI in that it includesa line relay and line cutoff relay and two message registers MRI andMR2. The register MRI is designed for operation by alternating currentof one frequency, for instance 20 cycles; and the register MR2 isdesigned for operation by alternating current of another frequency, forinstance, 60 cycles. There are no gas-iilled tubes controlling theoperative connection of the registers to conductor 33 as in linecircuits LCI, LC2 and LC3, the registers being directly and operativelyconnected through condensers I'I and 21 to this conductor. Y

Each of the message rate trunks MRTI and MRT2 includes party identifyingmeans comprising a test control relay 4I, a test relay 42, and a relay43 for controlling the frequency of the alternating potential applied tothe sleeve conductor of a connected line to effect selective operationof the calling subscribers register. Relay 4I is operated, immediatelyafter the first digit is dialed at the calling station, in the mannerdescribed in Patent No. 1,823,688 granted to H. Hovland, September 15,1931, to connect test relay 42 to the conductors of the calling line. Inthe case of a call initiatedat a station PI, relay 42 does not operate;but in the case of a call initiated at a station P2, relay 42 isoperated in series with the ringer 20, thereby closing a circuit foroperating relay 43. Relay 43 locks and remains operated until theconnection is released at the calling station. Each of trunks MRTI andMRT2 further includes a booster battery BB and two sources ofalternating potential GI and G2 of different low frequencies. For usewith line circuits LCI and LC2 of Figs. 1 and 2, the

frequencies may be 60 cycles for effecting breakdown of tube TI, andcycles for effecting breakdown of tube T2; for use with line circuit LC3in Fig. 3, the frequencies may be 750 cycles and 1500 cycles; and foruse with line circuit LC4 in Fig. 4, the frequencies may be 20 cyclesand 60 cycles. In trunk circuit MRT2, generators Gl and G2 are connecteddirectly in series with booster battery BB; while in trunk circuit MRTI,a transformer 44 is provided for superimposing an alternating potentialof the desired frequency on the booster battery.

The selector SI, associated With a line finder LF in each of thearrangements, is operatively controlled by the impulses created by thedialing of the first digit of a called subscribers number to select acorresponding group of terminals and to select an unguarded set ofterminals therein leading to a succeeding switch through which thedesired conversation may be established. Succeeding digits dialed by thecalling subscriber effect the operation of intermediate selectors, ifany, and a connector C2 to complete the connection with the called line.The connectionv is maintained under the control of the calling stationby the connection of ground potential in the connector C2 to the sleeveterminal in the bank of selector SI, the selector SI, line finder LFI,and cut-off relay all being held operated by this ground potential inusual manner. The connector C2 is arranged to ring the called station;and when the call is answered, the connector reverses the currentthrough the windings of the polarized supervisory relay to effect itsoperation. Relay 50 closes a circuit for operating the start magnet 5Iof timing mechanism TA, which mechanism may be individual to a trunkMRTI or MRT2, or common to a plurality of trunks. As soon as the timingmechanism moves out of normal position to start the timing of aninterval for which one operation of the calling subscribers messageregister is required, contacts 52 close a circuit for operating relay46. In trunk circuit MRTI the operation of relay 46 `disconnects theholding ground potential from conductor 49 and connects the boosterbattery BB, in series with the left winding of transformer 44 throughconductors 49 and 34, and a front contact of relay 35 to conductor 33,thereby impressing both the booster battery BB and an alternatingpotential of the desired frequency on conductor 33. In trunk circuitMRT2 the operation of relay 46 disconnects the holding ground potentialfrom conductor 49 and connects the booster battery BB in series with theone or the other of generators GI and G2, through conductors 49 and 34,to conductor 33.

Referring now to the line circuit LCI and trunk circuit MRTI, thecurrent through condenser 36, inductance 31 and winding 38 uponapplication of an alternating potential of a rst frequency fromgenerator GI through transformer 44 to conductor 49 and thence toconductor 33, generates odd harmonics in coil 38, a desired one of theharmonics being applied, by the tuned circuit comprising condenser I4and the primary winding of transformer I3 to cause breakdown of tube TI;the simultaneous application of booster battery potential to conductor33 being effective to maintain the operation of cut-oi relay 35, tomaintain the energization of tube TI and to effect the operation ofregister MRI. When relay 46 releases, ground potential is reconnected toconductor 49 to hold relay 35. In similar manner the application of analternating potential of z r a second frequency from generator G2,through transformer 44, to conductor 49 and thence to conductor 33,generates odd harmonics in coil 38, a desired one of the harmonics beingapplied, by the tuned circuit comprising condenser 24 and the primarywinding of transformer 23, to cause breakdown of tube T2; thesimultaneous application of booster battery potential to conductor 33being effective to maintain the operation of cutoff relay 35, tomaintain the energization of tube T2, and to eiect the operation ofregister MR2. Each succeeding closure of timing contact 52, While thetalking circuit is maintained, effects a reoperation of relay 46 and areoperation of the calling subscribers message register.

Referring now to the line circuit LC4 and trunk circuit MRT2, theoperation of relay 46 connects the one or the other of generators GI andG2 in series with booster battery BB through conductor 49, conductor 34,front Contact of relay 35, conductor 33, thence through condenser I 'Iand register MRI in parallel with condenser 2'I and register MR2. If thecall is from station PI and relay 43 is normal, the generator GI is soconnected, resulting in the operation of register MRI; but if the callis from station P2 and relay 43 is operated, the generator G2 is soconnected, resulting in the operation of register MR2. In either case,the connection of booster battery potential to conductor 34 maintainsthe energization of cut-off relay 35. When relay 4B releases, the meteroperating generator and booster battery are disconnected from conductor49, ground potential being reconnected thereto to hold cut-off relay 35.Each succeeding operation of relay 43, due to the periodic closure oftiming contact 52 while the talking connection is maintained effects areoperation of the calling subscriber's register. i

Referring now to line circuit LC2, the application of an alternatingpotential of a rst frequency, for instance, 60 cycles, to conductor 33causes the generation of odd harmonics in coil 33, a desired one of theharmonics being applied, by the tuned circuit comprising condenser I4and the primary winding of transformer I3, to cause breakdown of tubeTI; and the simultaneous application oi booster battery potential toconductor 33 is effective to maintain the energication of tube Ti, t-ooperate register MRI and to hold the line cut-oil relay 35. In likemanner, the application of an alternating potential of a secondfrequency, for instance, cycles, to conductor 33 causes the generationor" odd harmonics in the coil 38, .a desired one of the harmonics beingapplied, by the tuned circuit comprising condenser 24 and the primarywinding of transformer 23, to cause breakdown of tube T2; and thesimultaneous application of booster battery potential to conductor 33 iseffective to maintain the energization of tube T2, to operate registerMR2, and to hold the line cut-off relay 35.

Referring now to line circuit LCS, the applicavtion of an alternatingpotential of a first frequency, for instance '750 cycles, to Conductor33 causes a large enough current through condenser I4 and the primarywinding of transformer I3 to induce a voltage across the secondarywinding of this transformer to break down tube TI, the current throughcondenser 24 and transformer 23 being insufficient to cause breakdown oftube T2; and the simultaneous application of booster battery potentialto conductor 33 is eiective to maintain the energiz-ation of tube TI,operate register MRI, and hold relay 35. In like manner the applicationof an alternating potential of a seco-nd frequency, for instance, 1500cycles, to conductor 33 causes a large enough current through condenser24 and the primary win-ding of transformer 23 to induce a Voltage acrossthe secondary winding of this transformer to break down tube T2, thecurrent through condenser I4 and transformer I3 being insuflicient tocause breakdown of tube TI and the simultaneous application of boosterbattery potential to conductor 33 is effective to maintain theenergization of tube T2, operate register MR2, and hold relay 35.

It is, of course, apparent that application of breakdown and meteroperatin-g potentials to conductor 33 of each of line circuits LCi, LC2,LC3

`and L04 may be effected in the manner provided in trunk circuit MRTI orin the manner provided in trunk circuit MRT2. Other changes may be madein either the line circuit or the message rate 'trunk circuit within thescope of this invention.

While the arrangements shown in the drawings contemplate a singleoperation of a calling subscribers register for each message unit, themessage rate trunks may be arranged to effect one or a plurality ofoperations of the register for each message unit, the number ofoperations depending on the relative locations of the calling and calledstations.

What is claimed is:

l. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twothree-element gas-filled tubes, the anode element of each tube beingconnected to said conductor, means responsive to an impressed potentialof one frequency on said conductor for breaking down one of said tubes,means responsive to an impressed potential of an-other frequency on saidconductor for breaking down the other of said tubes, and two messageregisters, one register connected in the anode-cathode circuit of one ofsaid tubes and the other register connected in the anode-cathode circuitof the other of said tubes, means for applyi-ng ground potential to saidconductor to operate said relay, and means for applying booster batterypotential in place of ground potential to said conductor and forsimultaneously applying to said conductor an alternati-ng potential ofthe one or the other of said frequencies to selectively effect breakdownof a desired one of said tubes, said booster battery po tential beingeffective to maintain the energza tion of said relay and the operatedone of said tubes and to operate the associated register.

2. In a telephone system, a line circuit ccmprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twomessage registers, two three-element gaslled tubes, the anode of eachtube being connected to said conductor, the cathode of one tube beingconnected to one of the registers and the cathode of the other tubebeing connected to the other of the registers, two transformers, thesecondary winding of one transformer connected across the controlelements of one tube and the secondary winding of the other transformerconnected across the control elements of the other tube, a harmonicgenerator connected to said conductor, said generator adapted togenerate harmonics of the one or the other of two frequencies uponapplication of an alternating potential of the one or the other of saidfrequencies to said conductor, a tuned circuit comprising a capacitorand the primary winding of o-ne of said transformers for applying to thecontrol elements of one tube a desired one of the generated harmonies ofone of said frequencies to effect breakdown of said one tube, and atuned circuit comprising a capacitor and the primary winding of theother transformer for applying to the control elements of the other tubea desired one of the generated harmonics of the other of saidfrequencies to effect breakdown of said other tube.

3. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twomessage registers, two three-element gasiilled tubes, the anode of eachtube being connected to said conductor, the cathode of'one tube beingconnected to one of the registers and the cathode of the other tubebeing connected to the other of the registers, two transformers, thesecondary winding of one transformer connected across the controlelements of one tube and the secondary winding of the other transformerconnected across the control elements of the other tube, a harmonicgenerator connected to said conductor, said generator adapted togenerate harmonics of the one or the other of two frequencies uponapplication of an alternating potential of the one or the other of saidfrequencies to said conductor, and two tuned circuits in parallel, onetuned circuit comprising a capacitor and the primary winding of one ofsaid transformers for applying a, desired one of the generated harmonicsof one of said frequencies to the control elementsv of one tube toeffect breakdown of said one tube in response to application of analternating potential of said first frequency to said conductor, thelother tuned circuit comprising another capacitor and the primarywinding of the other of said transformers for applying a desired one ofthe generated harmonics of the other of said frequencies to the controlelements of the other tube to effect breakdown of said other tube inresponse to application of an alternating potential of said seco-ndfrequency to said conductor.

4. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twomessage registers, two three-element gaslled tubes, the anode of eachtube being connected to said conductor, the cathode of one tube beingconnected to one of the registers and the cathode of the other tubebeing connected to the other of the registers, two transformers, thesecondary winding of one transformer connected across the controlelements of one tube and the secondary winding of the other transformerconnected across the control elements of the other tube, a harmonicgenerator comprising an inductor connected to said conductor, saidgenerator adapted to generate harmonics of either one or the other oftwo frequencies upon application of an alternating potential of the oneor the other of said frequencies to said conductor, and two tunedcircuits in series connected in parallel with said inductor, one tunedcircuit comprising a capacitor and the primary winding of one of saidtransformers for applying a desired one of the generated harmonics ofone of said frequencies to the control elements of one tube to effectbreakdown of said one of said tubes in response to application of analternating potential of said one of said frequencies to said conductor,and the other tuned circuit comprising a capacitor and the primarywinding of the other of said transformers for applying a desired one ofthe generated harmonics of the other of said frequencies to the controlelements of the other tube to effect breakdown of the other tube inresponse to application of an alternating potential of said otherfrequency to said conductor.

5. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twomessage registers, two three-element gaslled tubes, the anode of eachtube being connected to said conductor, the cathode of one tube beingconnected to one of the registers and the cathode of the other tubebeing connected to the other of the registers, two transformers, thesecondary winding of one transformer connected across the controlelements of one tube and the secondary winding of the other transformerconnected across the control elements of the other tube, a harmonicgenerator comprising an inductor connected to said conductor, saidgenerator adapted to generate harmonics of the one `or the other of twofrequencies upon application of an alternating potential of the one orthe other of said frequencies to said conductor, a tuned circuitcomprising a capacitor and the primary winding of one of saidtransformers for applying a desired one of the generated harmonies ofone of said frequencies to the control elements of one tube to effectbreakdown of said one tube upon application of an alternating potentialof said one frequency to said conductor, and a tuned circuit comprisinganother capacitor and the primary winding of the other of saidtransformers for applying a desired one of the generated harmonics ofthe other of said frequencies to the control elements of the other tubeto effect breakdown of said other tube upon application of analternating potential of said other frequency to said conductor, andmeans for selectively applying to said conductor an alternatingpotential of either of said frequencies to' said conductor'to energizesaid generator.

6. In a telephone system, a line circuit comprising a con-trolconductor, a line cut-off relay, means connecting said relay to saidconductor, two message registers, two three-element gaslilled tubes, theanode of each tube being connected to said conductor, the cathode of onetube being connected to one of the registers and the cathode of theother tube being connected to the other of the registers, twotransformers, the secondary winding of one transformer connected acrossthe control elements of one tube and the secondary winding of the othertransformer connected across the control elements of the other tube, aharmonic generator connected to said conductor, said generator adaptedto generate harmonics of the one or the other of two frequencies uponapplication of an alternating potential of the one or the other of saidfrequencies to said conductor, a tuned circuit comprising a capacitorand the primary winding of one of said transformers for applying adesired one of the generated harmonics of one of said frequencies to thecontrol elements of one tube to effect breakdown of said one tube uponapplication of an alternating potential of said one frequency to saidconductor, and a tuned circuit comprising another capacitor and theprimary winding of the other of said transformers for applying a desiredone of the generated harmonics of the other frequency to the controlelements of the other tube to effect breakdown of said other tube uponapplication of an alternating potential of said other frequency to saidconductor, means applying ground potential to said conductor to operateand hold said relay, and means for selectively applying in place of saidground potential an alternating potential of the one or the other offrequencies to energize said generator.

'7. In a telephone system, a line circuit comprising a controlconductor, a line cut-off relay, means connecting said relay to saidconductor, two message registers, two three-element gasiilled tubes, theanode of each tube being connected to said conductor, the cathode of onetube being connected to one of the registers and the ede of the othertube being connected to the of the registers, two transformers, the secgof one transformer connected ntrol elements of one tube and the windingof the ether transformer consi; the control elements of the other aharmonic generator connected to said u: or, generator adapted togenerate of the one or the other of two freencies upon application of analternating potential of the one or the other of said frequencies tosaid conductor, a tuned circuit comprising a capacitor and the primarywinding of one of said transformers for applying a desired one of thegenerated harmonics of one of said frequencies to the control elementsof one tube to effect breakdown of said one tube upon application of analternating potential of said one frequency to said conductor, and atuned circuit comprising another capacitor and the primary winding ofthe other transformer for applying a desired one of the generatedharmonics of the other of said frequencies to the control elements ofthe other tube to effect breakdown of said other tube upon applicationof an alternating potential of said other frequency to said conductor,means applying ground potential to said conductor to operate and holdsaid relay, means for selectively applying in place of said groundpotential an alternating potential of the one or the other of saidfrequencies to energize said generator, and means for superimposing onthe alternating potential applied to said conductor a booster batterypotential for holding said relay operated.

8. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay, means connecting said relay to said conductor, twomessage registers, two three-element gas-filled tubes, the anode of eachtube being connected to one of the registers and the cathode of theother tube being connected to the other of the registers, twotransformers, the secondary Winding of one transformer connected acrossthe control elements of one tube and the secondary winding of the othertransformer connected across the control elements of the other tube, aharmonic generator connected to said conductor, said generator adaptedto generate harmonics of the one or the other of two frequencies uponapplication of an alternating potential of the one or the other of saidfrequencies to said conductor, a tuned circuit comprising a capacitorand the primary winding of one of said transformers for applying adesired one of the generated harmonics of one of said frequencies to thecontrol elements of one tube to effect breakdown of said one tube uponapplication of an alternating potential of said one frequency to saidconductor, and a tuned circuit comprising another capacitor and theprimary winding of the other transformer for applying a desired one ofthe generated harmonics of the other of said frequencies to the controlelements of the other tube to effect breakdown of said other tube uponapplication of an alternating potential of said other frequency to saidconductor, means for applying a potential to said conductor to energizesaid relay, means for selectively applying to said conductor to energizesaid generator an alternating potential of either of said frequencies inplace of ground potential, and means for simultaneously applying to saidconductor a potential effective to hold said relay operated and operatethe register associated with the energized tu-be.

9. In a telephone system, a line circuit comprising a control conductor,a line cut-off relay and two message registers, means connecting saidrelay to said conductor, means comprising a contact of said relay and acondenser connecting one of the registers to said conductor, and meanscomprising said contact and another condenser connecting the other ofthe registers to said conductor, one of the registers and the associatedcondenser Abeing tuned to render said register operatively responsiveonly to the application of an alternating potential of a rst frequencyto said conductor, the other of the registers and associated condenserbeing tuned to render this register operatively responsive only to theapplication of an alternating potential of a second frequency to saidconductor.

10. In a telephone system, a rst line, two subscribers stationsconnected thereto, a line circuit for said line comprising a controlconductor, a line cut-olf relay, means connecting said relay to saidconductor, two message registers, one forY each of said stations, meansincluding a Contact of said relay operatively connecting one of saidregisters to said conductor, and means including said contactoperatively connecting the other of `said registers to said conductor, asecond line and a station connected thereto, means comprising a messagerate trunk circuit connecting said lines y for conversation between acalling one of said two stations and a station on said second line,means connecting ground potential to said conductor to operate and holdsaid relay, said trunk circuit comprising means responsive to answer ofthe call at the called station for measuring off intervals of timeduring which the talking connection is maintained, each of saidintervals constituting a unit for which one operation of the callingsubscribers register is required, a booster battery, a generator of arst frequency, a generator of a second frequency, and means in saidtrunk circuit effective at the beginning of a measured interval forselectively connecting to said conductor a potential derived from theone or the other of said generators to operate the one or the other ofsaid registers as desired and for simultaneously connecting boosterbattery to said conductor in place of ground potential, the boosterbattery potential being effective to hold the relay operated while theregister is being operated.

EDWARD VROOM.

